A sliding gate, which is attached to the bottom of the ladle is used to control the flow rate of molten steel flow when molten metal, particularly molten steel, is poured from a ladle into a tundish in the continuous casting of steel.
FIG. 5 shows a sliding gate 200 attached to the bottom of a ladle 100. A so-called collector nozzle 2 is disposed on the lower side of this sliding nozzle 200. A long nozzle is a very important functional member used for preventing molten steel from being oxidized by atmospheric air and also for preventing molten steel from splashing when molten steel is poured from the collector nozzle 2 into a tundish (not shown).
When molten steel is poured from the ladle 100 into the tundish, molten steel flows down rapidly through a long nozzle inner hole. Through the dynamic pressure of this molten steel, the pressure in the inner hole becomes negative with respect to the outside atmospheric air. Due to this pressure difference, atmospheric air is sucked into the long nozzle inner hole through a fitting portion between the long nozzle and the collector nozzle. The sucked-in air oxidizes the molten steel flowing down through the long nozzle inner hole. As a result, qualities, such as cleanliness of the cast steel and the yield thereof, are remarkably lowered.
To solve the above problem, that is, to prevent molten metal from being oxidized by the air sucked through the fitting portion, methods have been carried out whereby 1) an inert gas such as argon gas or nitrogen gas is blown around the fitting portion or 2) a sealing material such as a refractory plastic material is used.
FIG. 6 shows an example of prior art, which was disclosed in Japanese Patent Laid-Open No. 1-100656 by the Applicant of this invention. A porous brick 50 is disposed at a fitting portion 7 between a head, which is reinforced by a metallic cover 16 of a long nozzle 3 and a collector nozzle 2. An inert gas is blown by passing through the porous brick 50 through an inert gas supply port 70, such that the air at the fitting portion and the upper and lower portions thereof is replaced with the inert gas. The pressure in the long nozzle inner hole is brought from a negative pressure to a pressure near the atmospheric pressure, by which the suction of air through the fitting portion is controlled and the sealing function is enhanced.
The above-conventional methods present various problems as described below, and results in a considerable problem in carrying out continuous casting stably and economically. In recent continuous casting, molten steel of 3 to 6 ladles has usually been cast continuously in one sequence cast, and further continuous-continuous casting (sequence continuous casting) of molten steel of 10 to 15 ladles has frequently been carried out.
In such continuous-continuous casting, the supply of molten steel from a first ladle to the tundish is carried out such that an allowable maximum amount of molten steel is supplied to the tundish. When the remaining molten steel in this ladle decreases, the first ladle is rapidly changed to a second ladle before the lower limit of tundish capacity is reached, and the supply of molten steel to the tundish is restarted.
Specifically, when the ladle is changed, the supply of molten steel to the tundish is finished, and after a sliding gate is closed, the collector nozzle of the sliding gate is separated from the long nozzle. As long as the long nozzle is sound even if the ladle is changed, the head of the long nozzle is cleaned to remove a sealing material or splashed steel around the head, and the collector nozzle of the next ladle is fitted to this long nozzle.
Before the collector nozzle of the next ladle is fitted to the long nozzle, a worker sometimes removes the remaining steel sticking to the fitting portion of the long nozzle through the use of an iron bar. Additionally, cleaning may also be performed by means of, for example, using oxygen gas to dissolve and remove the metal, which is sticking and solidifying to the lower part of the fitting portion. Through such repair work, the surface of the fitting portion 7 of the long nozzle gets rough. As the change frequency of the ladle increases, this phenomenon proceeds, and finally, any smoothness in the fitting portion is lost and the sealing function in fitting the long nozzle to the collector nozzle is impaired.
Thereupon, a limit to the use of the long nozzle itself is reached, and the quality and yield of the cast steel are lowered. The porous brick 50, which is integrally fixed to the long nozzle by means of a less permeable refractory mortar disposed at the fitting portion 7, is subjected to heat cycle of heating and cooling by the aforementioned work.
Therefore, various problems arise in that the head of the long nozzle is destroyed by thermal stress and physical stress, the blowing mode of inert gas is disturbed, and the sealing property is greatly impaired, such that the inflow of air into the long nozzle inner hole is allowed. These problems can be avoided to some degree, for example, by increasing the blowing amount of inert gas supplied to and around the fitting portion between the collector nozzle and the long nozzle through the porous brick. However, if the blowing amount of inert gas is increased too much, the inert gas blown into the long nozzle inner hole flows down directly into the tundish, resulting in a rampage of molten steel.
Consequently, slag and powder, which are a protective layer for the molten steel surface, are broken by the violent rampage of molten steel, such that the molten steel is exposed to the atmospheric air, or a large amount of molten steel is splashed. As a result, the quality of molten steel is degraded, or a safety problem arises. These problems cannot be overcome even when the blowing amount of inert gas is controlled.